1. Field of the Invention
The present invention relates to sawing and singulating semiconductor devices prior to a final assembly step into or onto a carrier, lead frame, substrate or Printed Circuit (PC) board. More particularly, the present invention relates to a method and a means for maintaining bonding pads or bumps ultra clean during a sawing or singulation process.
2. Description of the Prior Art
Heretofore, dicing saws fitted with thin dicing saw blades have been employed to cut streets or kerfs between individual die on a wafer mounted on a dicing tape or NITTO(trademark) tape named for the largest supplier of dicing tapes. The sawing operation creates very fine silicon dust that deposits onto the die and covers the bonding pads or electrodes on the die. Even though the state of the art dicing saws employ cooling water and cleaning water during the sawing operation it is necessary to wash the wafer in a wafer washer after sawing the wafer to remove silicon dust that collects in the recessed bonding pads.
Cooling water is sprayed onto the wafer and/or the saw blade at the kerf being cut. Even though deionized (D.I.) water is used for cutting and washing the wafer, friction can cause electrostatic discharge (ESD) that in turn may be great enough to damage state of the art semiconductor devices, but also causes poor wetting of the silicon dust which results in dry spot contamination of the bonding pads. Unless the silicon dust is completely removed wire bonding and bond reliability problems will result.
It is known that silicon dust particles build up on the blade of the wafer. These particles cause blade loading which shortens blade life and also causes chipping on the bottom of the wafer where the moving blade is exiting or leaving the cut.
D.I. water available at semiconductor plants has a conductivity level of 16 to 18 Meg Ohms per centimeter. If processed in house by CO2 known techniques and steps, ultra clean D.I. water can be produced which measures up to 1 (one) Meg Ohm per centimeter, and reduces ESD. This special very clean D.I. water comes with a cost. A state of the art dicing saw uses fifty gallons per hour. A wafer washer uses three to ten gallons of D.I. water per hour. The wash cycle and cooling water may be varied to reduce the dust spots on the bonding pads. It is possible to add surfactant wetting and cleaning solution to the wash and cooling water to achieve greater than 95% dust free bonding pads and to use less D.I. water.
It would be highly desirable to provide a method and means for eliminating the need for most if not all high cost ultra clean D.I. water used in dicing saws and wafer washers and at the same time increasing the dust free percentage area on bonding pads so as to enhance the strength of wire bonds.
It is a primary object of the present invention to provide a semiconductor wafer with virtually dust free surfaces and bonding pads.
It is a primary object of the present invention to provide a novel method of preventing silicon dust build up on wafers and bonding pads of die.
It is a primary object of the present invention to eliminate the need for all or most of all ultra-clean D.I. water and the use of CO2 processing of D.I. water.
It is a primary object of the present invention to provide a method which virtually eliminates die chipping at the bottom surface of a wafer.
It is a primary object of the present invention to provide a water-soluble removable non-ionic protective film for wafers during sawing operations.
It is a primary object of the present invention to provide an electrostatic discharge protective coating over conductive bonding pads and the wafer during sawing operations.
It is a primary object of the present invention to provide a removable water-soluble non-ionic film for silicon wafers.
It is a general object of the present invention to provide a method for virtually eliminating the need for D.I. water when performing sawing operations.
It is a general object of the present invention to provide a method which requires less very clean D.I. water when performing wafer washing operations and virtually eliminates the need for ultra-clean D.I. water.
According to these and other objects of the present invention clean wafers ready to be diced are coated with a removable water-soluble non-ionic film made from a gel, paste, or spray of a viscous liquid that is hardened by heat such as ultraviolet, infrared, or convection. The hardened film is non-soluble in cool water used for dicing, but is removable after dicing in a wafer washer using warm water and high water pressure. The protective coating virtually eliminates wafer dust deposits in the recessed bonding pads of the sawn die.